Yttrium-iron-garnet tuning elements are commonly used in electronically tunable band pass filters employed in microwave devices. In broad band microwave instruments such as receivers and spectrum analyzers these tunable band pass filters are often used in or before RF input circuits as tunable preselectors to insure that only a narrow band of frequencies is being supplied to the instrument input at any one time. This preselection is particularly useful in a broad band heterodyne instrument since it helps eliminate confusing spurious intermediate frequency (IF) signals due to the mixing of the local oscillator signal with input signals other than the desired one.
Prior art preselectors are housed in separate enclosures that are connected to subsequent circuit devices, such as mixers, by a length of transmission line. These prior art systems, however, suffer from a lack of flatness, i.e., there are variations in the signal level over the tuning range of the instrument. The variations in signal level arise because the YIG element reflects signals that are outside of its pass band as well as a small portion of the signals it is designed to pass. A mixer likewise reflects some of the signals that are incident upon it. The reflected signals present in the transmission line coupling a preselector with a mixer add or subtract to form standing waves, depending upon the frequencies of the signals present and the length of the line. The usual prior art solution to such variations in flatness is to provide some attenuation in the transmission line to absorb some of the reflected signals. This additional attenuation however increases the system noise figure undesirably.